5 Whys

5 Whys is an iterative question-asking technique used to explore the cause-and-effect relationships underlying a particular problem.[1] The primary goal of the technique is to determine the root cause of a defect or problem. (The "5" in the name derives from an empirical observation on the number of iterations typically required to resolve the problem.)

It is important to remember that the answer to each question then forms the basis of the next question. A disciplined approach to this principle ensures a logical flow as one digs deeper into finding the root cause(s).

Why? - The alternator belt was well beyond its useful service life and not replaced. (fourth why)

Why? - The vehicle was not maintained according to the recommended service schedule. (fifth why, a root cause)

The questioning for this example could be taken further to a sixth, seventh, or higher level, but five iterations of asking why is generally sufficient to get to a root cause. The key is to encourage the trouble-shooter to avoid assumptions and logic traps and instead trace the chain of causality in direct increments from the effect through any layers of abstraction to a root cause that still has some connection to the original problem. Note that, in this example, the fifth why suggests a broken process or an alterable behaviour, which is indicative of reaching the root-cause level.

It is interesting to note that the last answer points to a process. This is one of the most important aspects in the 5 Why approach - the real root cause should point toward a process that is not working well or does not exist.[2] Untrained facilitators will often observe that answers seem to point towards classical answers such as not enough time, not enough investments, or not enough manpower. These answers may be true, but they are out of our control. Therefore, instead of asking the question why?, ask why did the process fail?

A key phrase to keep in mind in any 5 Why exercise is "people do not fail, processes do".

The technique was originally developed by Sakichi Toyoda and was used within the Toyota Motor Corporation during the evolution of its manufacturing methodologies. It is a critical component of problem-solving training, delivered as part of the induction into the Toyota Production System. The architect of the Toyota Production System, Taiichi Ohno, described the 5 Whys method as "the basis of Toyota's scientific approach . . . by repeating why five times, the nature of the problem as well as its solution becomes clear."[3] The tool has seen widespread use beyond Toyota, and is now used within Kaizen, lean manufacturing, and Six Sigma.

There are two primary techniques used to perform 5 Whys:[4] the fishbone (or Ishikawa) diagram and a tabular format.[5] These tools allow for analysis to be branched in order to provide multiple root causes.

While the 5 Whys is a powerful tool for engineers or technically savvy individuals to help get to the true causes of problems, it has been criticized by Teruyuki Minoura, former managing director of global purchasing for Toyota, as being too basic a tool to analyze root causes to the depth that is needed to ensure that they are fixed.[6] Reasons for this criticism include:

Tendency for investigators to stop at symptoms rather than going on to lower-level root causes.

Inability to go beyond the investigator's current knowledge - cannot find causes that they do not already know.

Lack of support to help the investigator ask the right "why" questions.

Results are not repeatable - different people using 5 Whys come up with different causes for the same problem.

Tendency to isolate a single root cause, whereas each question could elicit many different root causes.

These can be significant problems when the method is applied through deduction only. On-the-spot verification of the answer to the current "why" question before proceeding to the next is recommended to avoid these issues. In addition, performing logical tests for necessity and sufficiency at each level can help avoid the selection of spurious causes and promote the consideration of multiple root causes.[7]